Vehicle battery current sensing system

ABSTRACT

A current sensing system, comprising at least one magnetic tunnel junction device placed adjacent to a current carrying conductor electrically connected to a battery of a vehicle. The magnetic tunnel junction device is configured to measure a magnetic field around the conductor. A monitoring device is operatively connected to the magnetic tunnel junction device, wherein the monitoring device is configured to receive the magnetic field measurement and determine an estimate of the current flowing through the conductor.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to and claims the priority benefit ofU.S. Provisional Patent Application Ser. No. 62/429,181, filed Dec. 2,2016, the contents of which are hereby incorporated by reference intheir entirety into this disclosure.

TECHNICAL FIELD

The present application relates to current sensing systems, and morespecifically, to a non-invasive vehicle battery current sensing system.

BACKGROUND

In recent years, electric vehicles (EVs) and hybrid-electric vehicles(HEVs) have entered a new era of much greater consumer acceptance.Compared to gasoline-powered cars, the reduced dependence on oil andlower emissions make them an attractive choice for the future of thetransportation industry. The electric power in an EV is provided by alarge series/parallel interconnection of rechargeable lithium-ionbatteries. To ensure reliable and efficient operation of the batteriesand early fault diagnosis, it is essential to have a simplified solutionfor predicting the health of various sub-systems in the vehicle. While,on one hand, the leakage current (a few milliamperes) between parallelconnected batteries can point to inefficient impedance matching, on theother hand, a sharp spike in current (a few 100s of Amperes) through thewires can be an indication of a short-circuit path requiring immediateuser attention. Hence, it is critical to have a unified solution whichcan non-invasively measure both DC and AC currents over a wide-rangewith a high-resolution.

There are several current measurement techniques proposed for EV/HEVapplications each with their own strengths and weaknesses. The shuntmethod is one of the most rudimentary methods of measuring the currentwhere the voltage drop across a resistor in series with the battery isused to calculate the current. Companies such as Texas Instruments andSENDYNE have proposed shunt sensor designs for automotive applicationswith galvanic isolation between the processing unit and the currentsensing circuit. However the shunt implementation is invasive and resultin significant power loss during high current operation. Hall Effectsensors are also popular in current measurement applications owing totheir low cost and the galvanic isolation they provide. However thesesensors are very sensitive magnetic fields and can be easily affected bystray fields inducing significant errors in small current measurements.Hall Effect sensors designed to measure small currents (<10 A)accurately must be shielded from stray fields and are invasive. Currenttransformers and Rogowski coils are current transducers that can operatein a wide frequency range. These devices are non-invasive, but can onlymeasure AC currents, and hence cannot be used in EV/HEV applications.Another non-invasive technology for current measurement is the flux-gatecurrent sensor. This sensor can measure down to low currents (˜50 mA)with a good dynamic range. However, flux-gate current sensors can becostly and bulky due to their complex magnetics and can have highself-heating due to large quiescent currents. Therefore, improvementsare needed in the field.

SUMMARY

According to one aspect, the present disclosure provides a non-invasivecurrent sensor comprising a TMR magnetic-field sensor, which utilizes aTunnel Magnetoresistance (TMR) effect in a Magnetic Tunnel Junction(MTJ) to generate a linear differential-output voltage proportional tothe magnetic field perpendicular to its package. An MTJ consists of athin insulator sandwiched between two ferromagnets. The direction of thetwo magnetizations of the ferromagnetic films can be changed by anexternal magnetic field. If the magnetizations are in a parallelorientation it is more likely that electrons will tunnel through theinsulating film than if they are in the oppositional (antiparallel)orientation. Hence as the orientation of the ferromagnetic layers changethe effective resistance across the device would also change.Consequently, such a junction can be smoothly transitioned acrossvarious states of resistance through the application of an externalmagnetic field. When installed on a current-carrying conductor of anelectric vehicle, the presently disclosed current sensor enablesmeasurement of currents ranging 10 mA-150 A with a resolution of 10 mA.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following description and drawings, identical reference numeralshave been used, where possible, to designate identical features that arecommon to the drawings.

FIG. 1(a) shows a TMR magnetic field sensor according to one embodiment.

FIG. 1(b) shows a schematic of the sensor of FIG. 1(a) according to oneembodiment.

FIG. 1(c) shows the response of an example TMR magnetic field sensor toan applied magnetic field in the range of +−50 Oe when the TMR2905 isbiased at 1V.

FIG. 2 shows a differential sensor arrangement according to oneembodiment.

FIG. 3 shows an example system diagram incorporating the sensors of FIG.1(a).

FIG. 4 shows the frequency response of the analog band-pass filter usingR₁=82Ω, R₂=200KΩ, C₁=C₂=1 nF.

FIG. 5 shows a process for sensing current through a conductor using thesensor of FIG. 1(c) according to one embodiment.

FIG. 6 shows an example current sensing system for mounting the sensorsof FIG. 1(c) according to one embodiment.

The attached drawings are for purposes of illustration and are notnecessarily to scale.

DETAILED DESCRIPTION

In the following description, some aspects will be described in termsthat would ordinarily be implemented as software programs. Those skilledin the art will readily recognize that the equivalent of such softwarecan also be constructed in hardware, firmware, or micro-code. Becausedata-manipulation algorithms and systems are well known, the presentdescription will be directed in particular to algorithms and systemsforming part of, or cooperating more directly with, systems and methodsdescribed herein. Other aspects of such algorithms and systems, andhardware or software for producing and otherwise processing the signalsinvolved therewith, not specifically shown or described herein, areselected from such systems, algorithms, components, and elements knownin the art. Given the systems and methods as described herein, softwarenot specifically shown, suggested, or described herein that is usefulfor implementation of any aspect is conventional and within the ordinaryskill in such arts.

FIG. 1(a) shows an example of a TMR magnetic field sensor 102 which isused as a current sensor according to one embodiment. FIG. 1(b) showsthe associated component schematic. As shown, the TMR may be implementedas a push-pull Wheatstone bridge configuration of four unshielded MTJelements 104 which act as magnetic-field dependent variable resistors asshown in FIG. 1 (b). The push-pull design provides a highly sensitivedifferential output that is linearly proportional to the magnetic fieldapplied perpendicular to the surface of the sensor package (alongz-axis) as shown in FIG. 1 (a). FIG. 1 (c) shows the response of the TMR(a TMR 2905 from Multidimension Technology Co., Ltd. in the illustratedexample) to an applied magnetic field in the range of +−50 Oe when theTMR2905 is biased at 1V.

To further improve the sensitivity of the TMR sensor 102, ahigh-amplification of the sensor output is needed. Experiments revealthat V₁≠V₂ of the sensor even under the absence of the magnetic field,i.e. there is an inherent offset in the differential-voltage. This callsfor an efficient offset-cancellation methodology, so that thedifferential voltage actually resulting from the external magnetic fieldcan be accurately measured. Further, for reliable and low-resolutioncurrent sensing, several noise-cancellation procedures are employed bothat analog and digital frontends. In one embodiment, to cancel anycommon-mode noise and interfering magnetic field, a differentialarrangement of two of the sensors 102 is provided as shown in FIG. 2 .As shown, the sensors 102 are placed 180° apart on a circular currentcarrying conductor 110 (with insulation 112) to enable differentialsensing of the magnetic field 108 produced by current in the conductor110 and efficient common-mode noise cancellation of the interferingexternal magnetic field 106. The sensors 102 may be also mounted againsta substrate 114 (e.g., a metal free PCB board) as shown to maintaintheir position relative to the conductor 110.

Assuming that the magnetic-field due to the current-carrying conductor110, at the location of the sensor 102, is B_(IN), and the totalexternal field is B_(ext) respectively, then the magnetic field measuredby each sensor can be written as:S _(1,input) =B _(IN) +B _(ext)S _(2,input) =−B _(IN) +B _(ext)

The output of the two TMR sensors with the applied magnetic field can bewritten as:S _(1,output)=(B _(IN) +B _(ext))C ₁S _(2,output)=(−B _(IN) +B _(ext))C ₂

Here, C₁ and C₂ incorporates the sensitivities of the two TMR sensorsand the gains of the analog front end. If the system is perfectlysymmetrical the values C₁ & C₂ will be identical giving a differentialoutput

${{\Delta\;{S\_}} = {{S_{1,{output}} - S_{2,{output}}} = {{2{CB}_{IN}} = {2{C\left( \frac{\mu_{0}I_{IN}}{2\pi\; r} \right)}}}}},{{{where}\mspace{14mu} C_{1}} = {C_{2} = C}}$$\begin{matrix}{\mspace{76mu}{{A\;\Delta\;{S\_}} = I_{IN}}} & (1)\end{matrix}$

Hence, the differential measurement rejects common mode noise, and strayfields (including Earth's magnetic field).

FIG. 3 shows an example architecture of the current sensing system 300according to one embodiment. The heart of sensing mechanism includes asensor 102 (e.g., TMR2905 magnetic-field sensor) placed in closeproximity to the current-carrying conductor 110.

This section demonstrates an exemplary implementation of the abovestated method for non-invasive, high-resolution sensing of DC and ACcurrents. FIG. 3 shows the experimental setup for each of the top andbottom sensors 102. Both top and bottom sensors 102 have the samearchitecture separately, except for the parallel orientation of sensor'sz-axis for efficient common-mode noise cancellation, as described above(FIG. 2 .). For simplicity, here we describe the current sensingmechanism for DC currents. The residual offset voltage in each of thetop and bottom TMR sensors is cancelled as follows.

Assume that the offset between V₊ and V⁻ is positive ΔV (ΔV=V₊−V⁻). Weup-convert the DC V₊ and V⁻ to 32.768 KHz by using analog Single-PoleDouble Throw (SPDT) switches driven by 32.768 KHz crystal. FIG. 3 showsa situation where bottom switches in both SPDT switches establish theconnection of 3.3V to the supply of TMR sensor, and V₊ output of thesensor to the input of the band-pass filter. In the next phase, theDAC-output connects to the input of the band-pass filter, therebyresulting in a square-wave (with rail-to-rail swing of ΔV) at the inputof band-pass filter. We choose multiple-feedback topology for theband-pass filter, centered at 32.768 KHz (f₀), to realize a high-Qfilter, thereby minimizing flicker noise from op-amp while alsorejecting undesirable high-frequency noise components. Forhigh-resolution current detection, the amplification ratio is set to˜450. FIG. 4 shows the frequency response of the analog band-pass filterusing R₁=82Ω, R₂=200KΩ, C₁=C₂=1 nF.

The resulting sine-wave at filter output is fed to 12-bit ADC operatingat 327.68 KHz (f_(s)=10f₀). The residual offset voltage, afteramplification by ˜450×, saturates the op-amp output. The DAC's outputvoltage is increased/decreased until V⁻ becomes close to V₊, i.e. untilthe offset is reduced sufficiently enough to result in a low-amplitudeunsaturated sine-wave at filter output. Now, any change in V₊−V⁻, due toexternal magnetic field, can be easily sensed by detecting the change insine-wave amplitude from its previous value.

To estimate the current flowing through the wire, we subtract thesampled values from the two oppositely placed sensors to get adifferential reading, thereby rejecting any common-mode noise, asdescribed in previous section. The resulting differential sine-wave iscross-correlated with an internally generated and stored digitalsine-wave of exactly same f₀ and f_(s). The equations governing theoptimal detection of the amplitude of differential sine-wave are asfollows:

$a = {\sum\limits_{1}^{N}{{x\lbrack n\rbrack}{\sin\left( {2\pi\frac{f_{o}}{f_{s}}n} \right)}}}$$b = {\sum\limits_{1}^{N}{{x\lbrack n\rbrack}{\cos\left( {2\pi\frac{f_{o}}{f_{s}}n} \right)}}}$${y = \sqrt{a^{2} + b^{2}}},$where f₀ is the sine-wave frequency (32.768 KHz), f_(s) is the samplingfrequency, N is the total number of samples in a computation.y gives the estimate of the amplitude which linearly relates to thecurrent flowing through the wire.

A flowchart describing the computational steps in the above exampleembodiment is shown in FIG. 5 . The process starts when the amplitude ofthe sine-wave through the analog-to-digital converter is read. Next, thesystem checks to see if the amplitude is saturated. If it is, the systemincreases or decreases the DAC analog output (e.g., about 3.3V) andrechecks the amplitude for saturation. Once an acceptable output isreached (non-saturation), the system cross-correlates the captured sincewave with internally stored sine-wayve to estimate the amplitude. Theamplitude is then used to determine the current flowing through theconductor 110 since the current linearly depends on the detectedamplitude. The current may then be displayed or otherwise received bythe vehicle control system to take appropriate remedial measures.

FIG. 6 shows one example implementation which includes two housingportions, each with a TMR sensor 102 embedded therein to allow the twosensors 102 to be mounted 180 degrees apart on a conductor wire.

In certain embodiments, four sensors 102 may be used, with two sensorson each side of the conductor, wherein the two sensors on one side aremounted orthogonal to each other. The interference can be furthercancelled by using a correlation amoung the measured outputs of the foursensors.

The sensors 102 and other components recited herein may include or beconnected to one or more computer processors and memory which arecommunicatively connected and programmed to perform the data processingand control functionality recited herein. The program code includescomputer program instructions that can be loaded into the processor, andthat, when loaded into processor cause functions, acts, or operationalsteps of various aspects herein to be performed by the processor.Computer program code for carrying out operations for various aspectsdescribed herein can be written in any combination of one or moreprogramming language(s), and can be loaded into memory for execution.The processors and memory may further be communicatively connected toexternal devices via a wired or wireless computer network for sendingand receiving data.

The invention is inclusive of combinations of the aspects describedherein. References to “a particular aspect” and the like refer tofeatures that are present in at least one aspect of the invention.Separate references to “an aspect” (or “embodiment”) or “particularaspects” or the like do not necessarily refer to the same aspect oraspects; however, such aspects are not mutually exclusive, unless soindicated or as are readily apparent to one of skill in the art. The useof singular or plural in referring to “method” or “methods” and the likeis not limiting. The word “or” is used in this disclosure in anon-exclusive sense, unless otherwise explicitly noted.

The invention has been described in detail with particular reference tocertain preferred aspects thereof, but it will be understood thatvariations, combinations, and modifications can be effected by a personof ordinary skill in the art within the spirit and scope of theinvention.

The invention claimed is:
 1. A current sensing system, comprising: afirst magnetic tunnel junction device placed adjacent to a currentcarrying conductor electrically connected to a battery of a vehicle, thefirst magnetic tunnel junction device configured to measure a magneticfield around the conductor and including a V₊ output and a V⁻ output; adigital to analog converter having a first input and a first output; afirst band-pass filter having a second input and a second output; afirst switching unit switchable from a first configuration whereat thefirst output is connected to the first magnetic tunnel junction deviceand the V⁻ output is connected to the second input, and a secondconfiguration whereat a voltage source is connected to the firstmagnetic tunnel junction device and the V₊ output is connected to thesecond input; and at least one processor operatively connected to thesecond output and the first input, wherein the at least one processor isconfigured to control the first output through the first input basedupon the second output.
 2. The current sensing system of claim 1,further comprising: a second magnetic tunnel junction device operativelyconnected to a second band pass filter through a second switching unit,the second magnetic tunnel junction device mounted 180 degrees apartfrom the first magnetic tunnel junction device with respect to theconductor.
 3. The current sensing system of claim 2, wherein the firstswitching unit is configured to provide an output at a frequency lowerthan a frequency of an analog to digital converter of the at least oneprocessor.
 4. The current sensing system of claim 1, further comprising:a second, third, and fourth magnetic tunnel junction device, wherein thefirst and the second magnetic tunnel junction device are mountedorthogonal to one another on a first side of the conductor, and thethird and the fourth magnetic tunnel junction device are mountedorthogonal to each other on a second side of the conductor opposite tothe first side.
 5. The current sensing system of claim 1, furthercomprising a wireless transmitter unit connected to the at least oneprocessor, wherein: the at least one processor is configured todetermine a current flowing through the current carrying conductor basedupon the measured magnetic field; and the transmitter unit is configuredto transmit data representing the determined current to a vehiclemonitoring system.